Current limiting fuse

ABSTRACT

A fuse structure characterized by a tubular electrically insulating casing having terminal means disposed at opposite ends thereof and having an axially extending, electrically insulating support member or core centrally disposed thereof with a fusible element mounted on the core in a helical path and connected between the terminal means. The core is a segmented member comprising three longitudinal sections, the end sections of which are low temperature electrically insulating members and a center section of which is a high temperature electrically insulating member.

United States Patent [191 Blewitt et al.

Appl

[58] Field Assignee:

Filed:

CURRENT LIMITING FUSE Inventors: Donald D. Blewitt, Pittsburgh;

Woodrow G. Shaw, Export, both of Pa.

Westinghouse Electric Corporation, Pittsburgh, Pa.

Oct. 17, 1973 US. Cl. 337/231, 333/159 Int. Cl. l'l0lh 85/16 of Search 337/159, 231, 280, 273, 337/276, 291, 294, 295

.References Cited UNITED STATES PATENTS 10/1965 Cameron 337/295 X 10/1971 Kozacka 337/159 5/1973 Mikulecky 337/159 X 1 Nov. 19, 1974 3,740,687 6/1973 Cameron 337/159 X 3,810,061 5/1974 Salzer 337/295 X Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or Firm-L. P. Johns [5 7 ABSTRACT A fuse structure characterized by a tubular electrically insulating casing having terminal means disposed at opposite ends thereof and having an axially extending, electrically insulating support member or core centrally disposed thereof with a fusible element mounted on the core in a helical path and connected between the terminal means. The core is a segmented member comprising three longitudinal sections, the end sections of which are low temperature electrically insulating members and a center section of which is a high temperature electrically insulating member.

8 Claims, 7 Drawing Figures CURRENT LIMITING FUSE CROSS-REFREENCE TO RELATED APPLICATIONS This invention is related to that disclosed in the application of Donald D. Blewitt, Ser. No. 407336, Filed Oct. 17, 1973.

BACKGROUND OF THE INVENTION I 1. Field of the Invention:

2. Description of the Prior Art:

The problem of core breakdown due to leakage currents in electric fuses has been the subject of investigation in an effort to determine the cause of certain fuse failures. Generally, breakdown occurs when the core supporting the fuse element is so badly damaged by the energy of interruption that a leakage current flows after interruption. The leakage current further degrades the insulating quality of the core until total breakdown occurs. The use of slots or longitudinal cutouts in the core as disclosed in the above-mentioned patent application is effective in extending the interrupting ability of the core. However, certain fuse ratings result in so much energy input to the core that even the use of slots is inadequate to prevent the breakdown.

As is well known in the art the low current clearing requirements for certain fuses necessitate that the element supporting core have some outgassing qualities to assist in the current interruption. The organic nature of the core material, however, may cause problems when required to sustain high energy inputs, such as those encountered when testing at the so-called critical current. In general, test models which were successful at current clearing would fail at'critical current, and those which would clear critical current would not interrupt at low current.

SUMMARY OF THE INVENTION It has been found in accordance with this invention that the foregoing problems may be overcome by providing a fuse structure comprising a generally tubular electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, and an axially extending electrically insulating support member-or core disposed in said casing with the end disposed adjacent to and spaced from the respective terminal means, a fusible elementdisposed in a helical path on the core and connected between said terminal means, the insulating support member being a segmented member comprising three longitudinal sections, the center section being composed of a ceramic material and the end sections being composed of a mixture of glass-fiber and polyester or melamine resin, and the adjacent sections being disposed substantially at right angles to each other.

The advantage of the fuse structure of this invention is that it provides for a high performance core structure which are not available with conventional corev structures.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a vertical sectional view of a current limiting fuse structure embodying the invention;

FIG. 2 is an elevational view of the core structure of the fuse shown in FIG. 1;

FIG. 3 is an end view of the core structure of FIG. 2 taken along the line III-III of FIG. 1;

FIG. 4 is a plan view of the core structure shown in FIG. 2;

FIG. 5 is a plan view of the center section of the core;

FIG. 6 is a fragmentary sectional view of another em bodiment of the invention; and

FIG. 7 is a wiring graph showing the manner in which the fuse may be tested.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 of the drawings a current limiting fuse structure is generally indicated at land is the type of fuse which is particularly adapted for a high voltage apv plication, such as 2.8 kv and above.'The fuse structure includes a generally tubular casing or housing 12, similar terminal means 14 at opposite ends of the casing, a fusible element 16, and a support member or core 18.

' axially projecting electrically conducting studs 22 are nal means 14 which are generally disposed between the opposite ends of the casing 12 and the respective ferrules 20. Each of the terminal means 14 is formed from a suitable electrically conducting material, such as copper or a copper alloy, and includes a central openingwith a tab portion formed integrally at one side of the central opening which projects axially inwardly at one end of the casing as indicated at 24.

In order to assist in supporting the fusible element 16 in the desired assembled configuration and to position the fusible element 16 inside the casing 12 at a location which is laterally or radially spaced from the inner bore of the casing 12 and for other purposes wbere desired, the core 18 is disposed axially inside the casing with the opposite ends of the core being disposed adjacent to and spaced axially from the respective terminal means 14. The support member or core 18 is preferably formed or molded from an electrically insulating material which has sufficient structural strength to withstand thermal conditions and pressures which may result during an interrupting operation of the fuse structure 12. As shown more particularly in FIGS. 2-5 the core 18 is a segmented member comprising a plurality of core pieces or sections 26, 28, and 30. The end pieces 26 and 30 are composed of a polyester or melamine resin reinforced with glass-fiber. An alumina tri-hydrate filler is added to evolve gas under are impingement. The end pieces 26 and 30 are preferably provided with slots or cutouts 32 extending longitudinally thereof and over a distance at least equal to one-half a turn of helically wound fusible element 16 in order to minimize current leakage paths which may otherwise result after burnback or vaporization of fusible element 16. The center piece 28 is composed of a high temperature, good quality electrical insulator, such as a high alumina ceramic material. At low current interruptions, the end pieces 26 and 30 evolve gas which effects current interruption. At high and critical currents, the center core piece 28, being composed of a higher temperature insulating material than the end pieces 26 and 30, is not damaged by the energy input and therefore it does not support leakage current. The center piece 28 also prerent rated fuse having a long time (low current) melting requirement, the core should have more gassing matecenter piece 28. For example, a fuse rating of 8.3 kv results in a core having a gassing high temperature ratio of about 1.6 to 1.

As shown in FIG. 3 the center piece 28, is disposed at substantially right angles to the end pieces 26 and 30 primarily to provide a rigid support member or core 18 for the fusible element 16. For example, the core pieces 26, 28, and 30 may be provided with suitable joint means such as longitudinal slots 34 (as shown for the center piece 28 in FIG. 5), which interfit with corresponding slots 35 in the end pieces 26 and 30 to provide a rigid core for the fusible element 16 as shown in FIG. 1. In order to provide proper spacing and placement of the helically wound fusible element 16 on the entire core 18, the core pieces 26, 28, and 30 are pro-, vided with similar edge notches 36 in which the element 16 is seated. Opposite ends of the fusible element 16 are in contact with the tab portions 24 by element end portions 38 which are secured between the corresponding pairs of ferrules 20 and terminal means 14.

The fusible element per se may be formed from a predetermined length of electrically conducting, fusible material, such as silver, of the flat, ribbon type and includes a plurality of axially spaced points of reduced cross sectional area as indicated at 40 which may be formed by V-notching the ribbon material from which the fusible element is formed on both sides of spaced locations along its length. The-notches 40 result in a series of restricted areas which fuse initially during an interrupting operation of the fuse structure 10 to provide a series of spaced arcs, whereby the sum of the voltages that cross said arcs result in a relatively high total are voltage during the operation of the fuse structure to limit the overload current which flows to a value less than that which would otherwise result during high fault current interruptions.

lsmeriLl s fi le witbataa smgr aranu arar quenching material 46, such as silicone sand or quartz sand, in which the fusible element 16 is embedded.

In order to evaluate the fuse structure 10 a leakage current monitoring circuit may be used to test the fuse as shown in FIG. 7. One test at a voltage (V) of 8.3 KVAC involved a critical current (I) of 4500A RMS closed at the fully asymmetrical condition on the current wave. Meter (M) is an inexpensive 0-10 ma meter shunted bya 500 ohms resistance for a full scale deflection of about ma AC. For testing the fuse, a current (I) of 4,500A was initiated, causing the fuse 10 to open almost immediately. After 10 cycles, normally closed circuit breaker 48 was opened to switch in the metering circuit. The 4,800 ohm resistor was used to limit the level of leakage current to less than 2 amperes and to provide a power factor near unity. This limiting resistor allows the fuse to fail gradually in an observable manner rather than suddenly and catastrophically. A deficient design will show an initially small leakage current which gradually increases in an erratic manner until the meter goes off scale. At this point, the fuse is generally smoking or afire and the meter has been burned out by the now-high value of leakage current. For safety reasons, the meter reading are monitored by a telescope located a safe distance from the test setup.

What is claimed is:

l. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, an axially-extending, electrically insulating support member disposed in said casing with the ends disposed adjacent to the respective terminal means, a fusible element disposed in a helical path on said support member and connected between said terminal means, the insulating 9299 1. membe bein ssamsnteimsrn qts mn is ing at least two connected longitudinal sections, one section being a high temperature electrically insulating material, and the other section being a low temperature electrically insulating material.

2. The fuse structure of claim 1 in which there are three longitudinal sections, one of the center and end sections being the high temperature electrically insulating member, and the other of the center and end sections being the low temperature electrically insulating member. y

3. The fuse structure of claim 2 in which the center section is composed of a ceramic material.

4. The fuse structure of claim 2 in which the end sections are composed of a mixture of glass-fiber and a resin selected from the group consisting of polyester and melamine.

5. The fuse structure of claim 4 in which the mixture also comprises alumina tri-hydrate tiller.

6. The fuse structure of claim 2 in which the center section is disposed at substantially right angles to'the end sections.

t 6 7. The fuse structure of claim 6 in which the joint betions consist of the high temperature electrically insutween the adjacent sectionsv comprise interfitting lating material, and the center section consists of the notches in the sections. low temperature electrically insulating material.

8. The fuse structure of claim 2 in which the end sec- 

1. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, an axially-extending, electrically insulating support member disposed in said casing with the ends disposed adjacent to the respective terminal means, a fusible element disposed in a helical path on said support member and connected between said terminal means, the insulating support member being a sgemented member comprising at least two connected longitudinal sections, one section being a high temperature electrically insulating material, and the other section being a low temperature electrically insulating material.
 2. The fuse structure of claim 1 in which there are three longitudinal sections, one of the center and end sections being the high temperature electrically insulating member, and the other of the center and end sections being the low temperature electrically insulating member.
 3. The fuse structure of claim 2 in which the center section is composed of a ceramic material.
 4. The fuse structure of claim 2 in which the end sections are composed of a mixture of glass-fiber and a resin selected from the group consisting of polyester and melamine.
 5. The fuse structure of claim 4 in which the mixture also comprises alumina tri-hydrate filler.
 6. The fuse structure of claim 2 in which the center section is disposed at substantially right angles to the end sections.
 7. The fuse structure of claim 6 in which the joint between the adjacent sections comprise interfitting notches in the sections.
 8. The fuse structure of claim 2 in which the end sections consist of the high temperature electrically insulating material, and the center section consists of the low temperature electrically insulating material. 